How to troubleshoot Mitsubishi PLC malfunctions based on the working status of LED indicators (POWER indicator, EPROR indicator, input indicator, output indicator). 01. POWER Indicator
The POWER indicator light, or power indicator, initially supplies power to the Mitsubishi PLC basic unit. Under normal circumstances, the POWER indicator light will illuminate and requires no adjustment. If it is not illuminated:
(1) Check if there is a problem with the power supply wiring. Another situation is when there is a drive sensor on the power supply, check for load short circuit or overcurrent, and adjust according to the actual situation.
(2) Check if there are any other conductive interferences or abnormal conditions inside the PLC that could cause the fuse in the basic unit to burn out. If so, remove the interferences and abnormal conditions, and then replace the fuse with a good one. EPROR indicator light 02
The EPROR indicator light, also known as the fault indicator light, is normally off. The EPROR indicator light may indicate the following faults:
(1) The EPROR indicator light is flashing. The reasons why the EPROR indicator light is flashing are:
① Program syntax error (such as forgetting to set the constant of the timer or counter).
② When abnormal noise, conductive foreign matter, or other reasons cause changes in the program memory content. If the Mitsubishi PLC is in the "STOP" state and all outputs are OFF, first check if there are any errors in the program, then check for conductive foreign matter or high-intensity noise sources.
(2) The EPROR indicator light is on. The reasons why the EPROR indicator light is on are:
①The first situation is that the CPU of the Mitsubishi PLC is affected by the introduction of conductive foreign matter or by the interference of abnormal external noise, causing the CPU to malfunction or the operation cycle to exceed 200ms, resulting in a WDT error and the EPROR indicator light to illuminate.
② Check if the PLC's grounding meets the requirements. If the process shows the [EPROR] LED changing from on to flashing, please check the program.
③ If the EPROR indicator light is constantly on, check if the program operation cycle is too long (the maximum scan time can be found by monitoring D8012).
④ If there is an internal problem with the PLC, please contact the technical service provider. If the PLC is in "STOP" mode and all outputs are OFF, you can perform a power-off reset. If the PLC returns to normal, please check for any abnormal noise sources and conductive foreign objects. 03 Input Indicator Lights
Common problems with input indicator lights:
(1) The reason why the circuit is conducting even though the input signal switch is in the OFF state:
① Poor contact can easily occur if the rated current capacity of the input switch is too large or if oil intrusion occurs.
② When the input switch and the LED light are connected in parallel with a resistor, even if the input switch is OFF, the parallel circuit is still conducting, and input can still be made to the Mitsubishi FX series PLC. The above reasons affect not only OFF inputs but also ON inputs.
(2) The reason why the circuit cannot be turned on even though the input signal switch is in the ON state:
① When the light-emitting/light-receiving parts of input devices such as optical sensors are disturbed by external factors (such as impurities, dust, or damage), the sensitivity drops significantly, resulting in a situation where the circuit cannot be formed.
② Different applied voltages at the input terminals can damage the input circuit. For example, this can cause the Mitsubishi FX2N series PLC to fail to receive ON and OFF inputs for a short period during its operating cycle. Output indicator 04
Output indicator light malfunction: The load cannot be ON or OFF. Possible causes:
① Overload, short circuit, or surge current from a capacity load can cause the relay output contacts to stick together.
② Poor contact surface at the contact point leads to poor contact.
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